This invention relates to an apparatus for cooling high-power density devices such as semiconductor devices and, more particularly, to a semiconductor liquid-impingement cooling module suitable for efficiently removing heat generated from semiconductor devices for use in a high-speed computer.
With the development of the technology for highly integrating semiconductor devices (elements) and highly densely packaging semiconductor devices (elements) on a base, various means have been studied to remove large amounts of heat generated from semiconductor devices. Semiconductor devices having a very large calorific value have recently been developed and, to cool such devices, a direct liquid cooling system to be used instead of forced convective air cooling systems or thermal conduction cooling systems has been proposed in which a semiconductor device is cooled by being directly immersed in a dielectric cooling medium so that heat is transferred by forced convection, forced convective boiling or pool boiling.
Cooling units based on a forced convective heat transfer system using a single phase liquid flow are known, which are, for example, one in which printed circuit boards on which a multiplicity of semiconductor devices are mounted are stacked and a cooling medium liquid is caused to flow between the printed circuit boards to cool the semiconductor devices (U.S. Pat. No. 4,590,538), and one in which an array of a multiplicity of heat sink fins are attached on the back side of semiconductor devices and a cooling medium liquid is caused to flow between the heat sink fins to cool the semiconductor devices (Japanese Patent Unexamined Publication No. 60-134451). In these cooling units, since semiconductor devices are arranged in one row in the direction along the cooling medium liquid flow, the temperature of the cooling medium liquid is increased in the flowing direction and the temperature of the semiconductor devices is gradually increased in the downstream direction, so that it is difficult to uniformly maintain the temperature of the semiconductor devices. This tendency is particularly conspicuous if the cooling medium used is an organic cooling medium having a small specific heat. However, cooling medium liquids available for use in these cooling units are needed to have suitable electric insulating performance and chemical stability and are limited to organic cooling liquids. In this type of cooling unit, therefore, the problem of non-uniformity of semiconductor device temperature is serious. Moreover, the thermal conductivity of organic liquids is very small, about 1/10 of that of water. It is therefore difficult to obtain a high forced convection heat transfer coefficient, and the efficiency of cooling semiconductor devices is necessarily low. To solve these problems, a type of cooling unit has been proposed (U.S. Pat. No. 5,021,924) in which nozzles are disposed in a one-to-one relationship with semiconductor devices in the vicinity of back surfaces thereof to cool the devices with a wall jet flow. Because a cooling medium liquid is separately supplied to each semiconductor device in this cooling unit, the non-uniformity of the temperatures of the semiconductor devices caused with increasing the liquid temperature can be reduced. Also, the thickness of velocity/temperature boundary layers can be reduced by the wall jet flow, so that a higher heat transfer coefficient can be obtained in comparison with the cooling units described above. In the design of this cooling unit, however, while the supply of the cooling medium liquid to each semiconductor device is achieved by disposing the nozzles in correspondence with the semiconductor devices, only the provision of the cooling medium liquid outlet port above each device has been considered with respect to discharge of the cooling medium liquid from each device. Therefore, the flow rate of the jet from a nozzle provided at the upstream portion of the discharged flow is necessarily smaller than the flow rate of another nozzle provided at the downstream portion because of a pressure loss effect. Also, no means is provided to completely discharge through each outlet port the cooling medium liquid supplied to the corresponding semiconductor device. A part of the cooling medium liquid supplied to a device placed at the upstream portion and warmed by this device flows to a device placed at the downstream portion, so that the temperature of the cooling medium liquid on the downstream portion is necessarily higher than the temperature on the upstream portion. These two unbalances cause non-uniformity of the performance of cooling the semiconductor devices and, hence, non-uniformity of the temperatures thereof. Although the heat transfer coefficient of a wall jet flow is higher than that of the ordinary internal flow, the former is at most 2 or 3 times higher than the latter. The heat loads which can be removed from semiconductor devices by using the above-mentioned organic liquid is at most 10 to 20 W/cm.sup.2 under the condition of an actually attainable liquid flow rate.
A pool boiling heat transfer type cooling unit is known as one of other major systems for cooling a high-power density devices. In this cooling unit, a group of devices are immersed in a pool of cooling medium at saturated condition and the devices are cooled by boiling of the liquid. For example, Japanese Patent Examined Publication No. 2-34183 discloses a cooling unit in which boiling enhancing fins having a multiplicity of fine cavities are attached to a back surface of a device, and boards on which a multiplicity of devices with such fins are attached are immersed together in a cooling medium liquid to cool the devices. This cooling unit achieves greatly improved device cooling performance in comparison with the above-mentioned force convection system because boiling enhancing fins are attached to the devices. In this cooling unit, however, it is necessary to remove the entire heat generated from the devices by vaporization of the cooling medium. The amount of vapor generated a the devices is therefore increased excessively, and a very large sectional area of a vapor flow path is required. The distance between the devices or between the device boards is thereby increased, and the problem of an increase in delay time of computing logical operation and a reduction in computation speed due to an increase in wiring distance is thereby encountered. Also, a large amount of vapor bubbles generated at each device pass around the other devices, and the boiling heat transfer coefficient is changed according to the amount of the bubbles, so that the temperatures of the devices are not uniform. Further, a vapor choking phenomenon occurs around some of the devices by concentration of vapor bubbles, and there is a risk of occurrence of dryout at that device. A cooling medium container for accommodating a multiplicity of boards must be a completely sealed container for maintaining the cooling medium at a saturated condition. There is therefore the problem such as a complicated structure for leading power supply or signal lines from the boards out of the container, a complicated sealing structure, and troublesome operations for repairing the devices on the boards.
A type of cooling unit using a combination of forced convection heat transfer and boiling heat transfer has also been provided. For example, Japanese Patent Unexamined Publication No. 2-22848 discloses a cooling system in which a multiplicity of devices are mounted in an inclined tubular path through which a cooling medium liquid is caused to flow downwardly along a slope while being boiled at each device. In this cooling unit, a cooling medium main flow direction component of the buoyancy of boiling bubbles generated at each device is directed to a direction just opposite to the direction of the main flow to direct the bubbles to a ceiling side of the tubular path remote from the base on which the devices are arranged, thereby reducing bubbles generated at upstream devices flowing into regions around downstream devices. A certain effect of this system has been confirmed. In this cooling unit, however, the amount of bubbles in the liquid flow in the vicinity of the base cannot be made uniform above the devices from the upstream to downstream positions, and occurrence of a situation where downstream devices are easily covered with the bubbles cannot be prevented if high-power density devices which can generate particularly large amount of bubbles are mounted. Moreover, since the effect of forced convective boiling heat transfer depends greatly upon the subcooled temperature of the liquid, the heat transfer coefficient of downstream devices is reduced in a structure in which downstream devices are cooled with the cooling medium liquid warmed by upstream devices. Because of the increase in the amount of bubbles and the reduction in heat transfer coefficient caused in this manner in the regions around downstream devices, the temperatures of the devices cannot be made uniform and the burnout heat flux corresponding to the limit of nucleate boiling cannot be sufficiently increased, so that the allowable heat loads of the devices, i.e., the amount of heat sufficiently removed by cooling cannot be increased. Japanese Utility Model Examined Publication No. 3-7960 discloses a cooling unit directed to the subject for increasing the allowable heat loads, in which impinging jet and boiling as combined. This cooling unit has a sealed cooling medium container which is formed of a section in which a board on which a plurality of semiconductor devices are arranged is immersed in a cooling medium liquid, and a cooling medium inlet header section from which the cooling medium is jetted to each device through nozzles. Boiling bubbles generated on a surface of each device are forcibly removed by the cooling medium liquid jetted through the nozzle. This cooling unit is thus arranged to improve the cooling performance and to cool devices having a heat load of about 10 W. Actually, in this cooling unit, boiling bubbles can be forcibly expelled from the device surfaces to improve the burnout heat flux and to increase the allowable heat loads. However, the influence of a flow of boiling bubbles generated from devices in lower positions to devices in upper positions has not been considered. Therefore an upward cooling medium flow containing bubbles warmed by lower devices flows into the regions of upper devices while increasing the liquid temperature and the amount of bubbles as it passes each device, so that the temperature of devices in upper positions is higher. If the heat loads of each device is large, the amount of bubbles around devices in upper positions is extremely increased to cause dryout at the devices, resulting in a reduction in the allowable heat loads. The problem of this temperature increasing of the upper devices and this reduction in cooling performance is serious with respect to present semiconductor devices having greater heat loads, i.e., 50 to 100 W devices. In addition the structure in which the whole board is immersed in the container has been adopted without considering leading a number of connection wires and signal wires on the order of several hundreds out of the container, the increase in logical operation delay time with the increase in the overall length of wires for wiring between a plurality of the containers or between the inside and the outside of each container, and maintenance facility with respect to device repairs and the like. Trials have been made to solve these problems and to avoid mutual interference between devices. For example, in HEAT TRANSFER IN ELECTRONICS 1989, ASME HTD-Vol. 111, pp 79 to 87 is proposed a cooling unit structure in which nine heaters used to simulate semiconductor devices are arranged in three rows and three columns on a base, and a cooling block having nine rectangular nozzles arranged in three rows and three columns so as to respectively face back surfaces of the heaters is provided on the base on which the heaters are mounted. The nozzle block in which nozzle orifices are formed is positioned with spacers provided above and below each row of nozzles so as to be spaced at a distance of 0.5 to 5 mm from the heaters. A restricted rectangular cooling medium flow path is formed between the nozzle block and each column of heaters. A cooling medium jetted from each nozzle to the surface of the corresponding heater impinges against the heater, changes its flowing direction through 90.degree., and flows through the rectangular flow path along a line tangent to the heater surface. Each spacer serves as a partition means between each adjacent pair of heaters to prevent mutual interference between the heaters in cooperation with partition plates extending longitudinally between the heaters. In this cooling unit, however, the cooling medium liquid jetted from each nozzle flows out of the flow path while being heated by the corresponding heater forming a part of the rectangular flow path and thereby generating bubbles. Therefore, the percentage of voids is increased toward the downstream end, and a vapor-liquid two-phase flow pattern is changed on the heater surfaces, so that a large temperature difference is produced between the heater surfaces. Also, since the flow path diameter for the two-phase flow is so small that flow pressure loss is very large. The pumping power for driving the cooling medium liquid is therefore increased and there is also a risk of the base being broken due to the increase of pressure on the heater. Further, as the cooling medium flowing out of each rectangular flow path in the two-phase state moves upward through the vertical flow path defined by the partition plates between the columns of the devices, the amount of bubbles in this flow generated by each heater is increased, so that bubbles generated by the heaters placed at the lower side influence on the upper side heaters, thereby causing differences between the temperatures of the heaters. In a situation similar to that described above wherein the vapor-liquid two-phase flow state varies with respect to the heater positions, if devices having different heat loads are used as in the case of an actual semiconductor device board the amount of bubbles generated at devices having a smaller heat load is small and the two-phase flow loss in the rectangular flow path is therefore small, while the flow loss is increased with respect to devices having a greater heat load, resulting in no-uniformity of distribution of the cooling medium liquid to the nozzles. Because of this non-uniformity, the flow rate is increased with respect to the lower heat loads devices with the smaller flow loss while the flow rate is reduced with respect to the higher heat loads devices the greater flow loss. This condition leads to a result contrary to an actual need for higher cooling performance for high heat leads devices.
A cooling unit also designed to reduce the mutual interference between devices is disclosed in Japanese Unexamined Patent Publication No. 2-237200. In this cooling unit, a multiplicity of countersunk holes formed in a wall of a liquid cooling medium header are placed above devices and the devices are separated from each other by these holes. In this cooling unit, however, cooling medium flow paths between the countersunk holes and the devices connecting device cooling cells and the cooling medium header are formed at a position offset from center axes of the device cooling cells such that the flow of the liquid cooling medium jetted through each nozzle cannot be formed as an axially symmetrical flow on the device surface. Owing to this asymmetry, considerable temperature non-uniformity is produced in each device and the cooling medium liquid stays at a device surface portion opposite to a cooling medium outlet position. The risk of occurrence of burnout from this portion is large. Owing to this asymmetry, as well, bubbles stay at ceiling portions of each device cooling cell outside the region where the cooling medium path is formed, thereby promoting dryout of the device cooling cell. A phenomenon can also occur in which formation of large bubbles and release of bubbles to the outside of each device cooling cell repeat alternately so that the cooling medium pressure in each device cooling device changes pulsatingly and so that the device temperature fluctuates. Further, contraction and enlargement of the flow path upstream and downstream of the cooling medium outlet may cause the flow loss of the cooling medium to be arbitrarily increased and make the vapor-liquid two-phase flow unstable, resulting in instability of the device temperature.
None of the above-described cooling units cannot solve all the problems of the variation in temperature in each device, difference between the temperatures of devices, the reduction in the allowable calorific value due to dryout caused by an increase in the amount of bubbles, the non-uniformity of distribution of the cooling medium liquid due to an unbalance of flow loss, the reduction in the allowable calorific value due to the reduction in the rate of liquid to some of the devices, and so on.
Also, the above-described conventional cooling units entail the problem of failure to limit variation in the temperature of each semiconductor device and the change in the pressure applied to the board because of instability of the two-phase flow.